A control unit for controlling executing operations of respective units (or modules) of a device configuring a multi-function device serving as a printer, a copying machine and a facsimile and incorporating functions thereof, has hitherto adopted a mechanism for outputting logs representing device internal states that include sensor values of the respective units in order to perform a test at a development and debugging, to conduct maintenance of a product on the market and to analyze a fault.
On the occasion of the product development, logs are invariably outputted to a host PC (Personal Computer) or the like connected to the device, and the development can be advanced while checking internal operations by analyzing these logs.
Then, for on-the-market machines, generally the logs are obtained only for periodic inspection by a serviceman and for taking urgent measures against a fault when such occurs. Therefore, what the periodic inspection can check is only a temporary state when in the inspection, and it is impossible to detect a latent fault that might not occur during the inspection.
Further, there is a possibility that when taking an urgent measure against a fault, the device internal state might already have changed, and the truly desired information, namely a picture of the machine state just when the fault occurs, cannot be acquired.
Moreover, the device internal state is merely left as a log while no consideration is given to a user's operation, and hence it is difficult to reenact an error and the fault when evaluating at the stage of the development and when taking the measure against the fault with respect to the on-the-market machine.
Such being the case, Japanese Patent Application Laid-Open No. 2002-283683 discloses a technology of enhancing the ability to reenact an error and a fault and facilitating the maintenance of the product and the fault analysis as well by recording the user's operation as a log in addition to the device internal state on a memory. Further, Japanese Patent Application Laid-Open No. 2002-283683 discloses also a technology of diagnosing the fault from these logs by the device itself.
According to the prior art described above, it is considered that the reenacting ability (i.e., the device's ability to reenact the error and the fault) and the reliability of self-diagnosis by the device are enhanced by taking as many detailed logs as possible.
As the number of logs gets larger, however, there is a higher possibility that the log recording process itself may restrict performance of execution of the normal control, that is, of the device's normal functions.
Further, it follows that logs unrelated to checking a target error and fault will increase in number, and therefore, conversely, it is difficult to select only the related logs from among these unrelated logs. In addition, the unrelated logs cause a futile expenditure of log storage area, and a period of real operation time of the device during which a history thereof can be recorded as a log, is reduced.
In these points, namely, a fundamental problem is that the recording target log is uniquely determined before the operation, but at that time it is not known which type of fault is going to occur during the operation or what the fault occurrence frequency is.